Looks like there's been a recent change to trunk such that x64 Windows
builds now get placed in pcbuild\amd64 instead of just pcbuild (thanks
to whoever added it). The attached patch against rt.bat allows it to
be called with an -x64 arg, which invokes the 64-bit Python build,
instead of the 32-bit one.
(tools/buildbot/test-amd64.bat should eventually be created to call
this as well, once we've ironed out all the Windows x64 issues.)